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I think you've made this thread in the wrong place. By talking about the episodes, I think you're refering to the TV show. This means that this thread is in the wrong forum and belongs in Pokemon TV Show and Movies, which I'll move it to.
It looks *sort* of like the thing scuba divers have https://en.wikipedia.org/wiki/Scuba_diving but it doesn't have any of the air supplies o.O I'm thinking it's supposed to be some sort of advanced thing that doesn't exist (maybe because they can't be bothered to draw the air supplies?) They don't have any of the rest of the equipment that you would probably need to be diving in deep lakes, so I'm supposing they just couldn't be bothered to draw it o.O
I really don't think that even exists. Remember the Pokemon world has teleportation abilities for ball capsules that contain energetically compressed biotic matter or something like that. Those are probably futuristic artificial gills.
The Pokemon world is a bit too advanced for us. xD Maybe its a sophisticated oxygen cylinder in which oxygen is very highly compressed. (Just my thought, its no where near true. xD)
I thought it looked like that underwater breather thing James Bond used in that 007 movie. I can't remember the name though... only remember that Sean Connery was Bond in it.